A donkey is a donkey by birth and it continues to be a donkey until it takes initiation of Vedas to become a horse:

Evidence 1:

Janmana Jayate Gardabhah, Samskarat Ashvah Uchyate (Skanda Purana)

By birth all animals are donkey (lowest animal), by the purification process (especially initiation by a homosexual or pedophile guru) a donkey(lowest animal) can become a horse(higher animal)

Evidence 2:

Kalau gardabhah sambhavah (Games of Thrones)

Every animal is born as a donkey in kaliyuga.